Why 4 degrees?

[digger]

Hiya's,
With the new generation engines...Ford coyote for example, 4 valve almost ultimate control...GM LT platform where you can advance or retard within 1/4 degree at will. How do you decide what change was better?
I datalog the MAF sensor as well as many other things

The dohc engines don't seem to be as sensitive to cam timing as older engines. The air flow is high enough things like overlap don't make much improvement.

i dont agree with this the late 90's BMW i6 stuff which have good 4V head on them made best output on about ~70 for the ECL and ICL at 2500rpm. you might think that thats to get the IVC and EVO correct for low rpm and the high overlap was along for the ride by virtue of fixed duration but you would quickly find without the correct collector length you could lose about 30% torque below 4500rpm so the overlap was also critical to moving air when the piston is not pulling its weight

[In-Tech]

Hiya's,
With the new generation engines...Ford coyote for example, 4 valve almost ultimate control...GM LT platform where you can advance or retard within 1/4 degree at will. How do you decide what change was better?
I datalog the MAF sensor as well as many other things

The dohc engines don't seem to be as sensitive to cam timing as older engines. The air flow is high enough things like overlap don't make much improvement.

i dont agree with this the late 90's BMW i6 stuff which have good 4V head on them made best output on about ~70 for the ECL and ICL at 2500rpm. you might think that thats to get the IVC and EVO correct for low rpm and the high overlap was along for the ride by virtue of fixed duration but you would quickly find without the correct collector length you could lose about 30% torque below 4500rpm so the overlap was also critical to moving air when the piston is not pulling its weight

Well, obviously you are not in control of the valve events. I spent a month on the electric dyno, a Whipple coyote and picked up, substantially everywhere, {Over 125whp at some points just datalogging MAF, MAP, RPM). Bone stocker with a supercharger and just moving valve events.

[Tom68]

This may be an elementary question but I never got a straight answer on this. Why do cam makers recommend installing 4 degrees advanced?

Here's a test on an engine with reasonable breathing (AFR headed 355), we have to assume it started from 4 degrees advance, so an additional 4 was crap and 4 less was ho hum unless you were chasing revs.

https://www.youtube.com/watch?v=nB2ERxT ... rdHoldener

[Stan Weiss]

This may be an elementary question but I never got a straight answer on this. Why do cam makers recommend installing 4 degrees advanced?

I don't know why they picked 4 degree. But if you look at the software I wrote with David Vizard or Rick Jones software. Those software packages tell you where to install the ICL for the cam they recommend. I would think if you order a cam from Mike he will also.

Stan

[Tuner]

The GM Power Manual published in the mid '70s states, "All Chevrolet camshafts are timed to give split overlap at 4 degrees before top dead canter. Both lifters should be equal lift above the base circle at 4 degrees BTDC."

This applies to all camshafts, production passenger and truck, and over the counter race cams, L88, ZL1, etc. I installed and measured many of them back in the day and with stock timing chain and sprockets they were all equal lift at 4* BTDC.

Why would they do this?

The equal lift point is the location of largest leak area between exhaust and intake.

[Tartilla]

I’m really curious as to why people are obsessed with overlap area and the triangle.

Not even being facetious, what are you guys seeing that you have been able to isolate this from changing events?

Who is obsessed? I gave you an example of not centering it. Of course, when you say 4 degrees advance is arbitrary it damn sure is.


Where the overlap triangle is makes a difference in the power curve.

I’d argue it only makes the tuned lengths more sensitive and opens up holes in the power curve in order to make more peak when the intake and exhaust are in tune. More overlap area just makes things more sensitive and too much just blows fuel out the exhaust that you would otherwise want to trap and use to work on the piston.

The opening and closing points dictate the power curve.

The amount of overlap that would blow fuel out the exhaust would differ on various engines and even head styles/port flow on the same engine.

Overlap is just a tuning method. Under-valved engines are going to need more overlap to create the same signal.

[Tartilla]

I just saw it as an advertisement for the LST and nothing more.

I'm a bit worried that with all Billy's valuable info....it's all still about the sales.

It absolutely is. Everything in this world is. It’s all for profit businesses, no one is doing this for charity, and it’s a dog eat dog world… cheap customers and competitors (American and foreign) trying to steal everyone’s work.

We all stand on the backs of giants.

That said, business ethics and care for the customer go a long way. Not many folks asking for charity.

On the other hand, I'm very grateful for the effort and time people devote here.

[Tartilla]

Just listening to Billy G saying more overlap for carb motors than fi motors because it gives the carb an early signal to pull fuel.

There's a lot of dynamic influence that is not always apparent.

Probably one of the reasons blower engines need way more carb cfm, as everything is isolated by the blower lobes.

[Tartilla]

This may be an elementary question but I never got a straight answer on this. Why do cam makers recommend installing 4 degrees advanced?

8 pages in. Did you get a straight answer yet (or even just an answer, straight or not)?

Several because it generally works well on a 2 valve wedge head. A bunch of other personal theories and heaps of barely related stuff.

Good thread though.

Oil and cam overlap thread almost always produce...lol.

[Tom68]

Overlap is just a tuning method. Under-valved engines are going to need more overlap to create the same signal.

More overlap reduces signal when the you don't have negative pressure in the exhaust port.

Under valved engines need more overlap because they need more duration to make power, in that case most of the rev range the overlap is simply trying to get the little valve open enough for when the piston starts to descend.

The GM Power Manual published in the mid '70s states, "All Chevrolet camshafts are timed to give split overlap at 4 degrees before top dead center. Both lifters should be equal lift above the base circle at 4 degrees BTDC."

That makes it very interesting.
Although on smaller cams we'll be reading that split overlap below 50 thou where ramps may not be symmetrical so the following may not apply.

So up until that time Chevs were only 4 degree advanced intake centerline when it's a single pattern cam symmetrical lobe.
I.E..A 220 220 110 with 4 degrees split overlap would be intake cl 106, = a +4 icl.
220 228 112 with 4 degrees split overlap would be intake cl 106, = a +6 icl.
220 238 114.5 with 4 degrees split overlap would be intake cl 106, = a +8.5 icl.

Makes sense for a single pattern and close to cams, gets a bit out there with increasing exhaust duration, but they were only using up to 10 degrees extra back then.

[Tom68]

Just listening to Billy G saying more overlap for carb motors than fi motors because it gives the carb an early signal to pull fuel.

There's a lot of dynamic influence that is not always apparent.

Probably one of the reasons blower engines need way more carb cfm, as everything is isolated by the blower lobes.

Billy no doubt talking about race motors with tuned exhausts within the rev range that they work. Big overlap is a mess everywhere else.

Blower engines are consuming more air, a 500hp sized NA carb is a restrictor on a 15psi blower app, it still only has atmospheric pressure on top of it.

[HQM383]

The GM Power Manual published in the mid '70s states, "All Chevrolet camshafts are timed to give split overlap at 4 degrees before top dead canter. Both lifters should be equal lift above the base circle at 4 degrees BTDC."

This applies to all camshafts, production passenger and truck, and over the counter race cams, L88, ZL1, etc. I installed and measured many of them back in the day and with stock timing chain and sprockets they were all equal lift at 4* BTDC.

Why would they do this?

The equal lift point is the location of largest leak area between exhaust and intake.

But why did GM choose 4* BTDC?

This is the OP’s original question isn’t it?

[In-Tech]

It was to start a 9 page thread about nothing

[Tuner]

Who is obsessed? I gave you an example of not centering it. Of course, when you say 4 degrees advance is arbitrary it damn sure is.


Where the overlap triangle is makes a difference in the power curve.

I’d argue it only makes the tuned lengths more sensitive and opens up holes in the power curve in order to make more peak when the intake and exhaust are in tune. More overlap area just makes things more sensitive and too much just blows fuel out the exhaust that you would otherwise want to trap and use to work on the piston.


The opening and closing points dictate the power curve.

The amount of overlap that would blow fuel out the exhaust would differ on various engines and even head styles/port flow on the same engine.

Overlap is just a tuning method. Under-valved engines are going to need more overlap to create the same signal.

Somebody said, "overlap that would blow fuel out the exhaust" ..

With a naturally aspirated engine the exhaust pressure is higher than the intake, so the exhaust is blowing into the intake. Seems like locating equal lift (maximum leak area) to minimize exhaust dilution is desirable.

[Tom68]

The GM Power Manual published in the mid '70s states, "All Chevrolet camshafts are timed to give split overlap at 4 degrees before top dead canter. Both lifters should be equal lift above the base circle at 4 degrees BTDC."

This applies to all camshafts, production passenger and truck, and over the counter race cams, L88, ZL1, etc. I installed and measured many of them back in the day and with stock timing chain and sprockets they were all equal lift at 4* BTDC.

Why would they do this?

The equal lift point is the location of largest leak area between exhaust and intake.

But why did GM choose 4* BTDC?

This is the OP’s original question isn’t it?

No, Wesman asked about 4 degrees advance, implying intake cl 4 degrees advance from equalised LSA angle install, because....

Screenshot 2024-01-28 122102.jpg

Tuner pointed out 4 degrees before (OVERLAP TDC) "equal lift" was considered an install point by GM presumably because cam advance has more to do with the overlap cycle than an intake highest lift point that is always around 40 degrees later than we want it anyways due to cam lobe practicality limitations.

It does seem nonetheless GM has headed more towards LSA plus around 4 degrees intake cl for intake later decades. Never mind the crazy emissions side tracks.

I hate these 360 crank degrees cam circles because there is 720 crank degrees of valve action.

unnamed.jpg

This is better. Except the degrees are woefully out of scale.

DegreeCamStep10.jpg